Purging-diluting method for removing gases



April 22, 1969 H. VEDDER 3,439,870

PURGING-DILUTING METHOD FOR REMOVING GASES Filed May 11, 1967 Sheet 1 Z of 2 April 2, 1969 H. VEDDER 3,439,870

PURGING-DILUTING METHOD FOR REMOVING GASES Filed May 11, 1967 Sheet 2 of 2 DUO- N meqwog IXIO -5TORR w a 04m ax 0' Tome 0 3O 6O .90 I20 I50 T 1 M E (mm) w uuw ko United States Patent 3,439,870 PURGING-DILUTING METHOD FOR REMOVING GASES I-Ielmut Vedder, Downers Grove, Ill., assignor to Chicago Bridge & Iron Company, Oak Brook, Ill., a corporation of Illinois Filed May 11, 1967, Ser. No. 637,694 Int. Cl. F04b 37/20 US. Cl. 230-69 13 Claims ABSTRACT OF THE DISCLOSURE A method of obtaining pressures of 1 10 torr or lower by reducing the pressure in a vessel to about 1 1()- torr after which a metal getter pump along with a gas purging step, wherein purging gas is introduced throughout the vessel to be evacuated, are employed to pump the vessel down to a pressure in the order of l torr or lower.

This invention relates to an improved method of obtaining pressures lower than l l0 torr and more specifically is concerned with a method of achieving extremely low pressures by the use of gas purging in combination with suitable evacuating means including mechanical pump means and a titanium sublimation pump.

Low pressure conditions are required for a number of purposes and programs. For example, it is necesary to simulate pressure conditions which will be encountered in carrying out space programs, e.g., testing equipment and personnel capabilities in outerspace. To conduct such programs, it is required that pressures in pressure vessels be on the order of l 1O torr and lower, a torr being equivalent to one millimeter of mercury.

With conventional mechanical pump systems, e.g., a rotary pump, pressures of only about 1 10 torr can be obtained, unless one desires to use two stage mechanical pumps or turbo-molecular pumps or two stage blowers, all of which are very expensive, in order to secure the required outputs. A known method to pump a vessel down to a pressure lower than 1 10- torr involves a combination of a mechanical pump and a liquid diffusion pump. However, a deficiency exists relative to the diffusion pump in that liquid used in the diffusion pump baekstrearns or enters the pressure vessel causing contamination in the pressure vessel and a shutdown of the pressure vessel until the liquid, such as oil, is cleaned out of the vessel.

Another system employed for reducing substantially the pressure in a vessel is the use of a titanium sublimation pump; however, it has been required to supplement this pump with another pump to achieve the desired pressure. This is because the titanium pump does not remove noble gases effectively. In the past, an ion pump has been employed to supplement the titanium sublimation pump to remove noble gases. The ion pump removes noble gases but it is not satisfactory because an ion pump is quite expensive and requires a great deal of time to remove noble gases and pump the vessel down to the desired pressure.

To overcome the disadvantages inherent with systems presently available for obtaining low pressure, I have invented a new and unobvious system in which pressures in the order of 1 10 torr and lower can be achieved at an optimum cost and in a short pump down time. My invention comprises a system wherein a mechanical pump is first employed for reducing the pressure in a vessel or container down to about 1X10- torr. A titanium sublimation pump is then utilized along with a gas purging step, which takes place either before or after the titanium pumping, to pump the vessel down "Ice to a pressure range in the order of 1 10- torr and lower.

When using the titanium sublimation pump, a number of the gases which remain following pumping with the mechanical pump are absorbed by the titanium; however, other gases such as the noble gases helium, argon, and krypton are not gettered by titanium and hence adversely affect lowering the pressure in a vessel or container to the desired level. Further, there is so much water condensed on all surfaces in the interior of a vessel that it takes much time for a pump of any kind to cope with the water vapor developed from this condensed water. I have discovered that if a purging operation is incorporated in the pumping system, the vessel can be pumped down to the desired level within a relatively short time and at a comparatively low cost. In the purging operation, a gas such as nitrogen, oxygen, carbon dioxide or the like, which contains the smallest amount of noble gases and water vapor, is supplied to a vessel which is at a reduced pressure. The purging gas mixes with the remaining gases in the vessel after which the mixed gases, including noble gases and water vapor, are removed whereby the vessel is able to achieve the desired pressure level in a short period of time and at a relatively low cost. The purging can be repeated as desired, each purging operation being effective to reduce the partial pressure of the noble gases and water vapor in the vessel. The benefits of utilizing my system are that a pressure of from about 1 l0 torr down to about 1 10 torr and lower can be achieved in a relatively short pumping time at a cost which is significantly reduced compared to other systems presently available for achieving extremely low pressures in a vessel.

Other advantages will be seen when reference is made to the remainder of the specification and the drawings.

FIGURE 1 shows a schematic diagram of a system utilizing a pumping arrangement in accordance with one embodiment of my invention;

FIGURE 2 shows a cutaway view of the spherical shaped container of FIGURE 1 illustrating the arrangement of the titanium sublimation pump;

FIGURE 3 shows a fragmentary view of a cooling plate arrangement employed in place of the cooling arrangement shown in FIGURE 2;

FIGURE 4 shows a further embodiment of my invention wherein a mechanical pump is utilized to remove the purging gas, residual gases and water vapor;

FIGURE 5 shows a curve illustrating the purging of a chamber with nitrogen in accordance with the instant invention; and,

FIGURE 6 shows curves A and B which pertain to the effect of the purging process disclosed herein.

Referring to the drawings, FIGURE 1 shows a system including a spherical tank 10 which is first evacuated or roughed down by means of a conventional mechanical pump means or system 13, such as a rotary pump and blower. Pump 13 assists in reducing the pressure in tank 10 to the order of 1X10 torr following which a titanium sublimation pump 14 can be employed to further reduce the pressure within the tank 10.

Titanium pump 14, illustrated in FIGURES 1 and 2, comprises cylindrical wall 16 and inner wall 17 spaced from wall 16 thereby forming an annular space 18 between the two walls. Liquid nitrogen or other suitable cooling medium is pumped from tank 19 through line 8 and circulated through space 18 for the purpose of cooling the surface of wall 17 after which the nitrogen passes through line 7 for reconditioning prior to returning to tank 19.

If desired, instead of the cooling structure exemplified in FIGURE 2, a cooling bafile arrangement as shown in FIGURE 3 can be employed. In this arrangement, the

cooling medium passes through line 7, cooling coils 5, and exits through line 8.

In operation, titanium filaments 20, fastened to removable plate 21, are heated for a short period whereby vaporized titanium from filaments 20 is deposited on wall 17 of pump 14. The gas components that make up air, e.g., nitrogen and oxygen, and which remain in tank 10 after the mechanical pumping step, are gettered by the vaporized titanium before and/or after it is deposited on wall 17. However, the noble gases in the air, such as helium, argon and krypton, are not gettered by titanium to any significant extent but remain as residual gases in tank 10. Moreover, water vapor which is developed from water condensed on all surfaces in the vessel when the pressure in the vessel decreases, can be considered as the main constitutent of the gas content of the vessel. The operation and construction of a titanium pump which can be utilized in my system is shown and described in greater detail in an article by R. W. Clausing entitled A Large- Scale Getter Pumping Experiment Using Vapor Deposited Titanium Films, Transactions of the Eighth Vacuum Symposium, 1961, page 345.

In an effort to eliminate the noble gases and water vapor from tank 10, a small amount of gas, such as a research grade nitrogen is released from tank 6 through line 4 into tank 10. By research grade is meant a nitrogen which is characterized by being extremely pure and having less than 5 parts per million of any contaminating gas, particularly helium, argon or krypton. A nitrogen gas which is commercially available for use in purging contains less than 5 parts per million of helium, less than 5 parts per million of argon, and is free of krypton and has a dew point of about l05 C. or 1 part per million of Water. This purging gas, which is extremely clean, mixes with the residual gases and offers an environment for the water vapor to mix with the purging gas. Additionally, because the purging gas is admitted into tank under pressure, the purging gas serves to force the water vapor off the inside wall of tank 10.

Following the admission of the purging gas, a turbomolecular pump 30 is actuated whereby the purging gas having residual gases and water vapor mixed therein is removed from tank 10 so that the partial pressure of the residual gases and water vapor in tank 10 is reduced. The turbo-molecular pump can be of the type made by The Welch Scientific Company, Skokie, Ill. and described in that companys bulletin entitled, New, Improved Turbo- Molecular Vacuum Pumps From Welch," No. 225'.21A- 3256. Subsequently, the pressure in tank 10 can be further reduced by actuating titanium pump 14 whereby the purging gas, nitrogen, is gettered to wall 17. If desired, in place of the turbo-molecular pump 20, a cold dilfusion pump of the type described in the article Cold Diffusion Pump, written by I D. Haygood, J. E. Nichols, and E. S. J. Wang, Advances in Cryogenic Engineering, volume 7, proceedings of 1961, can be employed for rfmoving the purging gas, residual gases and water vapor.

The purging operation can be repeated in the manner described above in order that the pressures of the residual gases, such as argon, krypton, etc. and water vapor are reduced as each purging operation removes a certain amount of the gases and vapor, not gettered by the titanium pump, in proportion to their partial pressures. Repeated purging aids in lowering the overall pressure in tank 10.

In the system embodied in FIGURE 4, tank 110 is first evacuated by means of a mechanical pump 113 in the same manner as described relative to the embodiment shown in FIGURE 1, such that the pressure in vessel 110 is reduced to about l 1O- torr. Subsequently, purging gas, such as nitrogen, is admitted to tank 110 from tank 119 to mix with the gases and water vapor remaining after the mechanical pumping operation thereby raising the pressure in tank 110 above 1 1O- torr. Mechanical pump 113 is then actuated to remove the mixed purging gas,

residual gases and water vapor, whereby the partial pressures exerted by the residual gases and water vapor is reduced. The purging gas need not be as clean or pure as the purging gas employed in the embodiment described relative to FIGURE 1. The steps of introducing the purging gas and removing the same with the residual gases and water vapor mixed therein can be repeated as desired until the tolerable pressure level exerted by the residual gases and water vapor in the system is reached. Subsequently, titanium sublimation pump 114, which operates in the manner described previously, is actuated and the gas components such as oxygen and nitrogen are gettered by titanium deposited on wall 117 whereby the pressure in tank is lowered to a level below 1X10 torr going as low as 1x10- torr and lower.

When purging occurs in the manner described with respect to FIGURE 4, a larger volume of purging gas is required than when purging occurs at a lower pressure level. Consequently, the more expensive, high garde research nitrogen gas utilized when purging at lower pressure levels is generally not used as the cost of reducing the pressure in the vessel in the embodiment described in FIGURE 4 would become prohibitive. However, if cost was not a pertinent factor and an extremely short pump down time was desired, research grade nitrogen could be employed in this embodiment. Thus, as seen in FIG- URE 4, line which delivers the purging gas to tank 110, is attached to the same source of nitrogen 119 which is utilized to deliver a cooling gas via line 126 to titanium sublimation pump 114.

In the discussion relating to purging in the systems described relative to FIGURES 1 and 4, nitrogen was set forth as the purging gas. Other gases such as oxygen, hydrogen, carbon dioxide, or vapors of organic fluids such as ethyl alcohol, methyl alcohol, ether, acetone, propane, butane, dichlorodifluoromethane, or dichlorotetrafluoroethane can also be utilized for purging the residual gases from a pressure vessel.

Thus, in one embodiment of the new and unobvious invention, low pressures in the order of 1 10- torr and lower are procured by first lowering the pressure in a tank or vessel by a mechanical pump means. The pressure is further reduced by means of a titanium sublimation pump. The remaining gases, including the noble gases and water vapor, which are not gettered by titanium, are purged from the vessel by introducing a purging gas, under pressure, into the vessel whereby the purging gas mixes with the residual gases. Subsequently, the mixed gases are removed from the vessel by means of a turbo molecular or cold diffusion pump such that the partial pressure of the noble gases and water vapor are lowered. By repeating the purging step, the residual gases, including noble gases and water vapor, are further diluted.

A further embodiment embraces the actuation of a mechanical pump means to first lower the pressure in a tank or vessel to about 1 10- torr. Then the purging gas is admitted to the vessel under pressure after which the mechanical pump is actuated to remove the purge gas having mixed therein gases and water vapor remaining in the vessel following the prior mechanical pumping. This step of purging and pumping can be repeated until the noble gases and water vapor, which are not effectively gettered by titanium, are diluted to a desired or tolerable pressure level. A titanium sublimation pump is then turned on and gases remaining in the vessel are gettered by titanium.

An illustration of the advantages of the new and unobvious method can be shown based on chamber evacuation tests which have been made. FIGURES 5 and 6 illustrate the pumping of a vessel down to a pressure range in the order of about 1 1O- torr.

Initially viewing the curve shown in FIGURE 5 where torr values are plotted with respect to the unit time in minutes, a mechanical pump was employed to pump an 18,000 liter chamber to 42 microns or 4.2 10 torr.

The chamber was then filled again with 2,000 microns or 2 torr, high purity (but not research grade) nitrogen. The chamber was then pumped down again with the mechanical pump to a pressure of about 43x10 torr. The steps of filling the chamber with nitrogen up to about 2 torr and pumping the vessel down again was repeated several times as seen in FIGURE 5, with the purging and diluting process taking approximately 90 minutes with the mechanical pump. For this particular experiment, no mechanical pump was immediately available, which was capable of further reducing the pressure in the chamber, so an oil dittusion pump, already available in the laboratory, was employed to further reduce the pressure in the chamber to about 3 microns or 3 X l torr. Thus, after about 95 minutes of the pumping and purging process, the chamber pressure was about 3 microns or 3 10 torr.

Subsequently a total surface area of about 12 square feet of a titanium sublimation pump was cooled by liquid nitrogen to about -196 C. after which 300 milligrams of titanium were sublimated on the 12 square ft. area of the pump. The nitrogen remaining in the chamber following the purging procedure discussed in relation to FIG- URE 5 was gettered by the titanium whereby the pressure in the chamber was reduced to approximately 7.5 torr after about 40 minutes, as seen in curve A of FIG- URE 6.

Curve B of FIGURE 6 illustrates a condition in which the 18,000 liter chamber had been evacuated by means of the mechanical pump and oil diifusion pump to 3 microns or 3X 10* torr. However, in the test illustrated by curve B, there was no purging of the chamber with nitrogen. Thus, more noble gases and water vapor were present at the 3 micron chamber pressure level than were present in the test conducted and illustrated by curve A. Upon operation of the titanium sublimation pump following the same procedure utilized relative to the test conducted which resulted in curve A, the pressure level in the chamber, as shown in curve B, was only about 1.5 10' torr after about 55 minutes as compared to the low torr range reached in only about 40 minutes in curve A. To further reduce the pressure in the test conducted relative to curve B, it was necessary to employ an ion getter pump having a pumping speed of 12 liters/ second for argon.

Although the purging diluting method of the present invention has been discussed with the use of a titanium sublimation pump, it is appreciated that other metal gettering pumps could be employed. Getteriug pumps which utilize metals which are readily vaporized such as zirconium, molybdenum, columbium, tantalum, calcium, magnesium, strontium, barium, aluminum, silicon lanthanum yttrium cerium, thorium, uranium and their alloys could be used.

The above described embodiments being exemplary only, it will be understood that the present invention contemplates structures diifering in detail from the presently described embodiments. Accordingly, the invention is not to be considered as limited, save as is constant with the scope of the following claims.

What is claimed is:

1. The method of procuring low pressures in a vessel in the order of 1 1O torr and lower in accordance with the steps of:

lowering the pressure in said vessel by mechanical pump means;

further reducing the pressure in said vessel by means of a metal getter pump;

introducing a purging gas throughout the entire volume of the vessel to be evacuated to mix with residual gases, including noble gases and water vapor, which remain in said vessel following pumping with at least said mechanical pump means; and,

removing said mixed purging and residual gases from said vessel to further reduce the pressure in said vessel.

2. The method of securing low pressures in a vessel in accordance with claim 1 wherein said purging steps of introducing purging gas followed by removing said mixed gases from said vessel are repeated.

3. The method of securing low pressure in a vessel in accordance with claim 1 wherein said mixed gases are removed from said vessel by means of a turbo-molecular pump.

4. The method of securing a low pressure in a vessel in accordance with claim 1 wherein said mixed gases are removed from said vessel 'by means of a cold diffusion pump.

5. The method of securing a low pressure in a vessel in accordance with claim 1 wherein said purging gas is nitrogen which is characterized in that it is extremely pure and contains less than 5 parts per million of helium, less than 5 parts per million of argon, and is free of krypton and has a dew point of about C.

6. The method of securing a low pressure in a vessel in accordance with claim 1 wherein said purging gas is in the group consisting of nitrogen, oxygen, hydrogen, carbon dioxide, and vapors of organic compounds selected from the group consisting of ethyl alcohol, methyl alcohol, ether, acetone, propane, butane, dichlorodifluoromethane and dichlorotetrafluoroethane.

7. The method of procuring low pressures in a vessel in the order of at least 1X1O torr in accordance with the steps of:

lowering the pressure in said vessel to about l 10- torr by mechanical pump means; further reducing the pressure throughout said vessel by means of a titanium sublimation pump;

introducing a purging gas in said vessel to mix with the residual gases in said vessel, including noble gases and water vapor;

removing said purge gas having said residual gases mixed therein by a pump means; and,

repeating said introducing and removing of purge gas.

8. The method of procuring low pressure in a vessel in the order of 1 10 torr and lower in accordance with the steps of:

reducing the pressure in said vessel by mechanical pump means;

introducing a purging gas throughout said vessel whereby said purging gas mixes with said remaining gases and water vapor in said vessel;

removing said mixed gases and water vapor to further reduce the pressure in said vessel; and

still further reducing the pressure in said vessel by means of a metal getter pump whereby gases remaining in said vessel following said removal are gettered by a vaporized metal.

9. The method of procuring low pressures in a vessel in accordance with claim 8 wherein said mixed gases and water vapor are removed from said vessel by said mechanical pump means.

10. The method of producing low pressures in a vessel in accordance with claim 8 wherein said steps of introducing said purging gas and removing said mixed gases and water vapor are repeated prior to reducing said pressure by means of said metal gettering pump.

11. The method of securing a low pressure in a vessel in accordance with claim 8 wherein said purging gas is in the group consisting of nitrogen, oxygen, hydrogen,

' carbon dioxide, and vapors of organic compounds selected from the group consisting of ethyl alcohol, methyl alcohol, ether, acetone, propane, butane, dichlorodifluoromethane and dichlorotetrafiuoroethane.

12. The method of procuring low pressure in a vessel to at least 1X10" torr in accordance with the steps of: reducing the pressure in said vessel to about 1X10- torr by mechanical pump means; introducing a purging gas throughout said vessel above l l0 torr whereby said purging gas mixes with said remaining gases and water vapor in said vessel;

removing said mixed gases and water vapor to reduce the pressure in said vessel to about 1 1()- torr thereby reducing the partial pressure of said remaining gases and water vapor; and still further reducing the pressure in said vessel by means of a titanium sublimation pump whereby gases remaining in said vessel are gettered by titanium and said pressure is reduced in said vessel to at least l 10 torr. 13. The method of securing a low pressure in a vessel in accordance with claim 12 wherein said purging gas 8 is nitrogen which is characterized in that it is extremely pure and contains less than 5 parts per million of helium, less than 5 parts per million of argon and is free of krypton and has a dew point of about 105 C.

References Cited UNITED STATES PATENTS 3,241,740 3/1966 Hamilton 230-69 3,252,652 5/1966 Trendelenburg et al. 230-69 ROBERT M. WALKER, Primary Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.0. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,439,870 April 22, 1969 He lmut Vedder It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 58, "constant" should read consonant Column 6, line 57, "producing" should read procuring Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents 

